Journal ofMaterials Chemistry A

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In situ synthesis

aState Key Laboratory of Silicon Materials, K

Application for Batteries of Zhejiang Provin

Engineering, Zhejiang University, Hangzhou

edu.cn; Fax: +86 571 87951152; Tel: +86 57bDepartment of Chemical and Biomolecula

College Park, MD 20742, USA. E-mail: cswa

† Electronic supplementary information(TGA) prole of the SnO2@C porous comporous composite aer being ultrasonigalvanostatic charge–discharge prolesnanoparticles and carbon black electro500 mA g�1. See DOI: 10.1039/c4ta04278f

Cite this: J. Mater. Chem. A, 2014, 2,18367

Received 18th August 2014Accepted 9th September 2014

DOI: 10.1039/c4ta04278f

www.rsc.org/MaterialsA

This journal is © The Royal Society of C

of SnO2 nanoparticlesencapsulated in micro/mesoporous carbon foam asa high-performance anode material for lithium ionbatteries†

Xiulin Fan,ab Jie Shao,a Xuezhang Xiao,a Xinhua Wang,a Shouquan Li,a Hongwei Ge,a

Lixin Chen*a and Chunsheng Wang*b

SnO2 has high capacity but poor cycling stability for Li-ion batteries due to pulverization and aggregation.

Herein, we tackle these two challenges by uniformly dispersing carbon coated nanoSnO2 into amicro-sized

porous carbon matrix to form a nano-SnO2/C composite anode using a facile and scalable in situ synthesis

strategy. The SnO2@C nanocomposite exhibits a capacity of 640 mA h g�1 at 500 mA g�1 in the initial 150

cycles and then increases to 720 mA h g�1 and maintains this capacity for 420 cycles. The superior

electrochemical performance with long cycle lifetimes of the carbon foam–SnO2 nanocomposites could

be attributed to their unique carbon microstructures: the network of carbon sheets provides favorable

electron transport, while the interconnected micro-/mesopores can serve as the effective channels of

lithium ion transport, thereby supplying short lithium ion diffusion pathways. Meanwhile, these pores

surrounding the active species of nanoSnO2 along with flexible carbon nanosheets can accommodate

the severe volume variations during prolonged electrochemical cycling and mitigate the Sn aggregation.

The present study provides a large-scale synthesis route to synthesize SnO2-based anode materials with

superior electrochemical performance for lithium ion batteries.

Introduction

Rechargeable lithium ion batteries (LIBs) have been extensivelyemployed as energy sources for portable electronic devices1 andare now showing promising applications in electric vehicles andgrid energy storage.2,3 As promising anode materials, tin-basedcompounds have been extensively investigated due to their hightheoretical capacities and low voltage plateau vs. Li+/Li.4–6

Among them, SnO2 is considered to be themost attractive anodematerial due to its high theoretical capacity of 790 mA h g�1.7

However, the poor cycling performance caused by serious Snparticle aggregation and considerable volume change uponcycling hampers its industrial applications.8 Besides, theintrinsic low electrical conductivity of SnO2 also reduces the rate

ey Laboratory of Advanced Materials and

ce, Department of Materials Science and

310027, PR China. E-mail: lxchen@zju.

1 87951152

r Engineering, University of Maryland,

ng@umd.edu

(ESI) available: A thermogravimetricposite, a TEM image of the SnO2@C

cated in ethanol for up to 6 h, andof the mixture of commercial SnO2

des at a constant current density of

hemistry 2014

performance. Several strategies have been reported to circum-vent these intractable issues. One effective protocol is to designSnO2 nanomaterials with different morphologies ranging from1D nanorods to 3D nanostructures,9–14 which can effectivelysustain the volume variation, and more effectively access to Li+

ions in the electrolytes.15 Another useful approach is toconstruct SnO2 nanocomposites with conducting materialssuch as amorphous carbon,16–18 graphene,19–26 carbon tubes,27

graphite28–30 and conducting polymers31 to improve theconductivity of SnO2 anodes, circumvent the aggregation andpulverization of Sn nanoparticles. Sun et al.32 deposited theSnO2 nanoparticles onto the graphene nanosheets using theatomic layer deposition (ALD) method. Wang et al.33 fabricatedgraphene–SnO2 sandwich structures, using GO and SnCl2 as theprecursors. Using the hard template method, Lou et al.18

designed a coaxial SnO2@carbon hollow sphere. These SnO2/carbon composites with precisely controlled structure exhibit ahigh capacity and long cycling life. However, complex andexpensive synthesis methods limit wide applications of thesematerials. To reduce the cost, Wang et al.34 fabricated theSnO2@C composite by a simple impregnation–calcinationmethod, but the SnO2@C composite provides a capacity of 730mA h g�1 in the 10th cycle and quickly drops to 590 mA h g�1 inthe 50th cycle. More recently, Jahel et al.35 synthesized a micro/mesoporous SnO2/C composite by impregnating tin salt

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solution into the porous carbon. The composite was reported topossess a reversible initial capacity of 916 mA h g�1, but thecapacity drops to maintain 560 mA h g�1 aer 100 cycles at arate of 1 C. The SnO2/C composites synthesized using facilemethods show unsatisfactory cycling stability due to theirinhomogeneous structure. The synthesis of a SnO2-based elec-trode with superior lithium storage performance using a scal-able and facile method still remains a signicant challenge.36

The ease of fabrication of electrode materials that can be massproduced at low cost is of great interest on an industrial scale.37

In this study, SnO2@C nanocomposites with superior electro-chemical performance were synthesized using a facile and scalablesolution–evaporation–calcination method (Scheme 1). The in situsynthesized composite exhibits a high capacity of 420 mA h g�1 at2000 mA g�1 and maintains 720 mA h g�1 at 500 mA g�1 for420 cycles, which is similar to or better than the SnO2/C compositesynthesized using very expensive so34,35,38 or hard templatemethods.39 The exceptional performance of the SnO2@C nano-composite is attributed to the unique structure. In the SnO2@Cnanocomposite, SnO2 nanoparticles uniformly dispersed in themicro/mesoporous carbon matrix, where the porous carbon caneffectively accommodate the volume change of SnO2 during lith-iation/delithiation, quickly transport the electrons and Li-ions, andmitigate the nano-Sn aggregation.

Experimental sectionMaterial synthesis

Stannous sulfate (SnSO4), sodium sulfate (Na2SO4), sulfuric acid(H2SO4) and sucrose (C12H22O11) were purchased from Sino-pharm Chemical Reagent Co., Ltd. SnO2 nanoparticles(�50 nm) were purchased from Aladdin Co. Ltd. All reagentswere of analytical grade and used as received without anyfurther purication. To prepare the carbon foam–nanoSnO2

composite, SnSO4 (12 g), carbon precursor of sucrose (12 g), andthe dispersant Na2SO4 (24 g) were dissolved in 100 mL ofdeionized water. Sulfuric acid (2 mL) was added to the solutionto suppress the hydrolysis of SnSO4 and catalyze the carbon-ization of sucrose. The resulting mixed solution was dried in adrying oven under severe stirring, followed by ball milling for 20min to obtain very ne powder. Aer that, the powder wascalcined at 400 �C for 2 h and subsequently at 600 �C for 4 h in atube furnace under an Ar atmosphere and then cooled to roomtemperature in the presence of Ar. The obtained black productwas washed with deionized water several times to dissolve

Scheme 1 Schematic illustration of the formation process of the SnO2@and calcination, respectively.

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Na2SO4 and then a pure carbon foam–nanoSnO2 (SnO2@C)composite was obtained.

Characterization

X-ray diffraction (XRD) experiments of the samples were per-formed on an X'Pert Pro X-ray diffractometer (PANalytical, theNetherlands) with Cu Ka radiation at 40 kV and 40 mA. X-rayphotoelectron spectroscopy (XPS) was carried out on a VGESCALAB MARK II system with Mg Ka radiation (1253.6 eV) at abase pressure of 1 � 10�8 Torr. All binding energy values werereferenced to the C 1s peak of carbon at 284.6 eV with anuncertainty of �0.2 eV. Transmission electron microscopy(TEM, Tecnai G2 F30) and scanning electron microscopy (SEM,Hitachi SU-70) were performed to examine the morphology andmicrostructure of the products. Pore size distributions, specicsurface areas and pore volumes of the samples were charac-terized by N2 adsorption by means of a Quantacrome Autosorb-1-C system using the Brunauer–Emmett–Teller (BET) and Bar-rett–Joyner–Halenda (BJH) methods.

Electrochemical investigation

The electrochemical tests were performed using a coin-type halfcell (CR 2025). Metallic lithium was used as the negative elec-trode. To prepare working electrodes, SnO2@C or nanoSnO2–CBmixture (70 : 25), carbon black (CB), styrene–butadiene rubber(SBR) and sodium carboxymethyl cellulose (CMC) with a massratio of 90 : 5 : 2.5 : 2.5 were mixed into a homogeneous slurryin deionized water with a pestle and mortar. The slurry mixturewas coated onto Cu foil and then dried at 80 �C for 12 h undervacuum. The electrolyte solution was comprised of 1 M LiPF6 inethylene carbonate (EC)/dimethyl carbonate (DMC)/ethylmethylcarbonate (EMC) (1 : 1 : 1 by volume). The cells were assembledwith a polypropylene (PP) microporous lm (Celgard 2300) asthe separator. Charge–discharge experiments were performedat a constant current density between 0.005 and 3 V using aLAND CT2001A Battery Cycler at room temperature. Cyclic vol-tammetry (CV) was performed on the CHI660E electrochemicalworkstation in the potential range of 0–3.0 V (vs. Li+/Li) at a scanrate of 0.1 mV s�1.

Results and discussion

In the present work, using SnSO4 as a SnO2 source and sucroseas carbon sources, and Na2SO4 as a dispersant, a facile solution-

C porous composite. Ev., BM and Ca. denote evaporation, ball milling

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evaporation–calcination method is used to in situ synthesize theSnO2@C porous composite. The strategy for synthesizing theSnO2@C composite is schematically depicted in Scheme 1. Thedetailed procedures are shown in the Experimental section.First, SnSO4, NaSO4 and sucrose are dissolved in water to form asolution and then evaporated. During the evaporation of thesolution, the sucrose will be polymerized into a polymerizationnetwork in the presence of sulfuric acid. At the same time, thehydrolysis of SnSO4 is suppressed by the sulfuric acid, resultingin a homogeneous dispersion of sulfate in the polymerizationnetwork. In the subsequent calcination, SnSO4 decomposes intoSnO2 nanocrystals and the polymerization network carbonizesinto carbon nanosheets. The presence of Na2SO4 can separateSnSO4 and therefore suppress the overgrowth of SnO2 nano-particles during SnSO4 decomposition. Ten grams of thecomposite can be prepared in a batch, and Na2SO4 can becollected and reused aer crystallization. The uniformlydistributed pores formed during the removal of Na2SO4 caneffectively accommodate the volume change, and accelerate iondiffusion in the SnO2@C electrode during lithiation/delithia-tion processes.

The phases of the SnO2@C porous composite were charac-terized by XRD. For comparison, the pattern of the commercialnanoSnO2 (�50 nm) is also shown. Fig. 1 shows the XRDpatterns of the SnO2@C porous composite and commercialnanoSnO2. All peaks in the XRD patterns of the composite canbe indexed to tetragonal SnO2 (JCPDS no. 41-1445). Comparedto the XRD patterns of the commercial nanoSnO2 samples, thewide diffraction peaks of the SnO2@C nanocomposites may beattributed to the small SnO2 particle sizes. The average grainsize of SnO2 nanoparticles in the composites is calculated fromthe three largest diffraction peaks (110), (101) and (211) by usingthe Scherrer equation (d ¼ 0.9l/b cos q), and the estimatedaverage crystal size is about 14.5 nm for SnO2 nanoparticles.

The carbon content in SnO2@C nanocomposites was deter-mined using thermogravimetric analysis (TGA) (Fig. S1†). Thefraction of nanoSnO2 in the composite is calculated to be 71.1%

Fig. 1 XRD patterns of the as-synthesized SnO2@C composite andcommercial SnO2 nanoparticles.

This journal is © The Royal Society of Chemistry 2014

by weight. The SnO2 percentage of the composite is higher thanthat of most of the composites synthesized by impregnationmethods,27,35 which ensures a higher reversible capacity for thecomposite.

The morphology of the SnO2@C composite was studiedusing transmission electron microscopy (TEM, Fig. 2a–c),scanning electron microscopy (SEM, Fig. 2d and e) and highresolution transmission electron microscopy (HRTEM, Fig. 2f).As shown in Fig. 2a, typical SnO2@C porous particles are inmicro-size with a diameter of 1.5 mm. In the magnied TEMimages (Fig. 2b and c), SnO2 nanoparticles are uniformlydistributed and entangled in the micro-sized carbon matrix.The carbon matrix is composed of carbon nanosheets, whichcan be clearly observed in Fig. 2a, b and e, denoted as CS in thegure. From Fig. 2a, b and e, it can be clearly seen that thenanoparticles of SnO2 are totally blanketed and separated by thecarbon sheets. The HRTEM image (Fig. 2f) demonstratesthat the SnO2 nanoparticles are in a single crystal with a size of10–20 nm, which is consistent with the XRD results. Latticefringes with a spacing of 0.26 nm can be seen from the HRTEMimage, corresponding to the (101) planes of SnO2. It is worthnoting that no visible change can be observed aer ultra-sonicating the dispersion solution of the SnO2@C composite inethanol for up to 6 h (Fig. S2†), which demonstrates the stronginteraction between the SnO2 and carbon sheets. The stronginteraction between SnO2 and carbon will greatly enhance thecharge–discharge cycling stability.

Compared to 2D composites, such as nanosheet supportedmaterials,40,41 3D nanoporous SnO2@C composite particles withmicro-size can better accommodate the volume change andrefrain the cracking, crumbling and SnO2 pulverization becausethe carbonmatrix with layers of carbon sheets can safeguard thein situ formed nanoSnO2 in all directions. In the case of twodimensional composites, the amorphous alloy aggregatesgenerated aer lithiation are easily peeled off from the surfaceof the supporters upon cycling because of the weak contactbetween the particle and supporters, therefore leading to a fastdecay of the electrode capacity.33,42

The structures of SnO2@C nanoporous composites werefurther characterized using a surface-sensitive high-resolutionX-ray photoelectron spectroscope (XPS).43 Fig. 3 shows thetypical full XPS spectra of the SnO2@C porous composite andcommercial nano-SnO2. The photoelectron lines at a bindingenergy of about 285 and 532 eV observed both in the SnO2@Cporous composite and commercial nano-SnO2 are attributed toC 1s and O 1s, respectively. Intensive peaks of Sn 3d5/2 and 3d3/2at �487 and �495 eV are clearly observed in commercial nano-SnO2 nanoparticles,44 but are almost absent for the SnO2@Cporous composite. Since the atomic sensitivity factor of Sn ismuch higher than those of C and O,45 the dramatic reduction insignals of Sn 3d5/2 and 3d3/2 implies that the in situ formed SnO2

nanoparticles in the SnO2@C nanoporous composite arecovered by carbon layers, which is in good agreement with theabove SEM and TEM observations. The carbon layer coating onnano-SnO2 can increase the electronic conductivity and preventnano-Sn aggregations during repeated lithiation/delithiation.

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Fig. 2 (a) TEM image of the SnO2@C porous composite. (b) and (c) are the magnified images of the border and interior of the composite,respectively. The inset of (b) shows the EDS result of the composite. The presence of Cu is derived from the copper mesh that supports thecarbon film with the sample during measurement. (d) and (e) are SEM images of the SnO2@C foam. (f) HRTEM image of the SnO2@C porouscomposite. CS denotes carbon sheets.

Fig. 3 XPS survey scans of (a) commercial SnO2 nanoparticles and (b)as-prepared SnO2@C porous composite.

Fig. 4 N2 adsorption/desorption isotherm of the Sn2O@C porous

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The porous structure of SnO2@C was determined by N2

isothermal adsorption/desorption measurements. Fig. 4 depictsthe N2 adsorption/desorption isotherm of the composite andthe corresponding pore size distribution determined by theBarrett–Joyner–Halenda (BJH) model (inset, Fig. 4). The BETspecic surface area is measured to be �326 m2 g�1, which ismuch higher than that of the composite synthesized by theimpregnation method.34 The large specic surface area wouldbe benecial to increase the contact with electrolyte and Lidiffusion upon lithiation/delithiation. The pore size

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distribution (inset of Fig. 4) shows that the composite exhibits amixed micro-/mesoporous structure. The micropores may comefrom the carbonization of sucrose due to the release of volatilespecies,46 while the mesopores arise from the dissolvation ofNa2SO4 aer calcination. Based on the BET data, the specicpore volume of SnO2@C is calculated to be 0.65 cm3 g�1. Themesopores associated with micropores can effectively accom-modate the severe volume changes during electrochemicallithiation/delithiation processes, which would increase thegeneral mechanical resistance of the composite matrix as awhole.

composite; the inset is the BJH pore size distribution curve.

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The electrochemical performance of the SnO2@C porouscomposite was evaluated using coin-type half cells andcompared to that of commercial nano-SnO2. Fig. 5a shows thecyclic voltammetry (CV) curves of the SnO2@C electrode in therst six cycles between 0.005 and 3 V vs. Li+/Li. A small lithiation(cathodic) shoulder at 0.7–0.95 V and a large peak at 0.1 V in therst cycle can be assigned to the irreversible formation of Li2O/SEI and a LixSn alloy, respectively. During the rst delithiation(anodic) process, the strong peak at 0.7 V and the broad peak at1.26 V correspond to the delithiation phase transition from theLixSn alloy to Sn.25 The CV curves of the following cycles almostoverlap, implying the excellent reversibility of the SnO2@Celectrode. Fig. 5b shows the charge–discharge proles of the

Fig. 5 (a) Cyclic voltammetry curves of the SnO2@C porouscomposite electrode at 0.1 mV s�1 scanning rate. (b) Galvanostaticcharge–discharge profiles of the SnO2@C electrode at a constantcurrent density of 500 mA g�1. (c) Cycling performance of as-synthesized SnO2@C and commercial nanoSnO2 with Cmixture (SnO2

& C) electrodes at 500 mA g�1.

This journal is © The Royal Society of Chemistry 2014

SnO2@C electrode at a current rate of 500 mA g�1 between 0.005and 3 V. A delithiation plateau at 0.7 V is observed in the rstdischarge of commercial SnO2 nanoparticles (Fig. S3†) andbecomes a slope plateau at 0.7–0.5 V for the SnO2@C nano-porous composite (Fig. 5b), which is due to the formation of SEIand the conversion of SnO2 to Li2O and Sn.33 This potentialplateau is signicantly reduced in the second lithiation,demonstrating the irreversible reaction of SEI formation andconversion reaction. The initial discharge and charge capacitiesof the SnO2@C composite are 1473 and 640 mA h g�1 with acoulombic efficiency of 43.4%. The relatively low initialcoulombic efficiency is ascribed to the irreversible conversion ofSnO2 into Sn and Li2O,47,48 and the formation of SEI lms49–51

with minor contribution from the irreversible insertion of Li+

into the micropores in the carbon matrix.35 The cycling perfor-mances of the SnO2@C composite and nanoSnO2 are shown inFig. 5c. The reversible capacity of the SnO2@C porous compositegradually increases from initial 640 mA h g�1 to more than720 mA h g�1 at the 200th cycle, and maintain 720 mA h g�1 for410 cycles. A volumetric capacity of 1100 mA h cm�3 is achievedfor SnO2@C. The behavior of capacity increase has beenreported for the Sn/C nanocomposite,38,52 SnO2 nano-composite53 and other metal oxide systems,54,55 which can beattributed to the reversible formation and decomposition of anorganic polymeric/gel-like layer from the electrolyte duringdischarge–charge cycling,52 or delithiation/lithiation of LiOH.56

The coulombic efficiency increases from initial 43.4% to >99.5%in ten cycles and 99.5% of coulombic efficiency is maintainedfor 410 cycles. In contrast, the capacity of commercial nanoSnO2

quickly drops from 650 mA h g�1 to 30 mA h g�1 in 40 cycles.The rapid capacity decline is also reported for many nanoSnO2

materials.15,17,57,58

The SnO2@C composite with the porous carbon matrixalso exhibits much exceptional rate capability, as shown inFig. 6. The SnO2@C porous composite can provide 730mA h g�1

at 200 mA g�1, 510 mA h g�1 at 1000 mA g�1, 420 mA h g�1 at2000 mA g�1, and 250 mA h g�1 even at a current density of4000 mA g�1, which is one of the best performances among the

Fig. 6 Rate capability of the SnO2@C porous composite at differentcurrent densities.

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Fig. 7 TEM (a and b) and SEM (c) images of the SnO2@C porous composite after the 100th electrochemical cycle. (d) is the EDS mapping resultsof the (c) image. The tremendous carbon signals outside the sample are derived from the carbon tape.

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reported SnO2/C composites.7 Remarkably, when the currentrate is reduced back to 200 mA g�1 aer more than 100 cycles,the initial capacity is completely recovered.

The excellent cycling stability and rate capability of theSnO2@C porous electrode might be attributed to the uniquenanoporous structure of micro-sized SnO2@C. Specically, theprimary island-like SnO2 nanoparticles possess a short distancefor Li+ ion diffusion and a large electrode–electrolyte contactarea for high Li+ ion ux across the interface, leading to theenhanced rate capability.15,59 The carbon sheets can sufficientlyact as the electron conducting channels during lithiation/deli-thiation processes and prevent the aggregation of Sn nano-particles. Meanwhile, the local empty space around the SnO2

nanoparticles associated with the carbon sheets can effectivelyaccommodate the large volume change, thus leading toimproved lithium-storage properties. The highly uniformnanoSnO2@Cmorphologic aspect allows full utilization of SnO2

nanoparticles to react with lithium ions, thus providing highcapacity and improving the cycling stability. It is worthmentioning that the capacity retention of the SnO2@C porouscomposite is signicantly enhanced when compared with manyother SnO2 composites59–61 due to the unique structure ofuniform dispersion of carbon coated SnO2 nanoparticles in themicro-sized porous carbon matrix.

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The structural and morphological changes of the SnO2@Cnanoporous composite aer 100 cycles were investigated usingTEM, SEM and EDS analyses. Fig. 7a and b present TEM imagesof the SnO2@C porous composite aer the 100th lithiation/delithiation cycle. No pulverization and agglomeration of thematerial are observed, which can explain the excellent cyclingstability of the SnO2@C porous composite. Fig. 7d shows theelemental mapping results of Fig. 7c for the distribution of Sn,O and C elements on the composite. As can be seen, the Sn andO signals are much stronger than that of C in the composite. Inaddition, the elemental mapping clearly indicates homoge-neously dispersed Sn, O and C. This homogeneity implies ahighly dispersed Sn distribution without any detectable aggre-gation, which will be favorable for lithiation/delithiationprocesses. The large amount of C outside the composite isderived from the double sided carbon tape which was used forSEM measurements.

Conclusions

In summary, we have synthesized a hierarchical SnO2@Ccomposite with carbon coated SnO2 nanoparticles uniformlydispersed in the micro-sized porous carbon matrix using afacile and scalable solution–evaporation–calcination method.

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The SnO2@C nanoporous composite can provide a capacity of420 mA h g�1 at 2000 mA g�1 and maintain 720 mA h g�1 at500 mA g�1 for 420 cycles. The exceptional performance isattributed to the unique SnO2/C nanoporous structure, wherethe uniformly dispersed carbon coated SnO2 nanoparticles inthe porous carbon matrix can (1) increase the electronic andionic transport in the electrode, (2) effectively accommodate thevolume variations of SnO2 during lithiation/delithiation, and (3)prevent Sn nanoparticles from aggregation, thus providing highrate performance and cycling stability. Combining a simple andscalable synthesis method with excellent electrochemicalperformance, the present strategy makes the SnO2@C porouscomposite a promising anode material for LIBs.

Acknowledgements

The authors gratefully acknowledge the support of the Programfor Innovative Research Team in University of Ministry ofEducation of China (IRT13037), the Key Science and TechnologyInnovation Team of Zhejiang Province (2010R50013), the ChinaPostdoctoral Science Foundation (2012M521167) and theInternational Postdoctoral Exchange Program (2013).

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